Method and system for vehicle ESC system using map data

ABSTRACT

An Electronic Stability Control (ESC) system for a vehicle is disclosed. An electronic control unit (ECU) is programmed to reduce vehicle lateral skidding by reducing differences between an intended vehicle yaw rate and an actual vehicle yaw rate by applying modifications to operation of the vehicle brakes and/or throttle. The ESC system receives inputs from wheel speed sensors, a steering wheel position sensor, a yaw rate sensor and a lateral acceleration sensor. The ECS system also receives input that indicates at least a property of the road upon which the vehicle is located, wherein the road upon which the vehicle is located is determined from a positioning system that uses a map database and the property is determined from the map database. The ESC system incorporates the road property information in determining when and/or how to modify operation of the vehicle to reduce vehicle skidding.

BACKGROUND OF THE INVENTION

The present invention relates to Electronic Stability Control systemsfor vehicles.

Electronic stability control (ESC) systems are a powerful safetyaddition to modern vehicles. ESC systems help drivers maintain controlunder compromising road conditions, challenging maneuvers and variationsin driver responses or abilities. Many vehicle models being manufacturedtoday have installed ESC systems as standard equipment. It is expectedthat ESC systems will become even more widely used in the future.

ESC systems are a computer-based technology that detects and reduceslateral vehicle skidding. An ESC system detects a loss of vehiclesteering control and then applies appropriate braking individually ateach wheel to help direct the vehicle in accordance with the driver'ssteering wheel input. More specifically, electronic stability control(ESC) systems operate by individually actuating the wheel brakes toinduce a yaw moment on the car for the purpose of improving stabilityand performance. In order for the ESC system to accomplish this, the ESCsystem analyzes what the car is actually doing, and what the car“should” be doing under ideal circumstances.

Conventional ESC instrumentation includes a steering wheel positionsensor, a lateral accelerometer, wheel speed sensors, a yaw rate sensor,sensors for brake and throttle inputs, and an enable/disable switch. Theactuation is managed by an electronic control unit (ECU) and a hydrauliccontrol unit (HCU). The ESC system can actuate wheel brake pressure aswell as engine throttle to affect the dynamics of the car.

ESC system design tries to assist the driver without either takingcontrol away from the driver or overriding the driver's “feel” of theroad. It is a feature of ESC systems that the driver should be able tofeel some slip of the tires to know when to self-modulate the speed orsteering. When the ESC system does activate, the character of thedriver's response does not change. The ESC system intervention makes thedriver's inputs more effective.

Although existing ESC systems provide useful benefits and advantages,there continues to be room for improvements.

SUMMARY OF THE INVENTION

In view of the above, systems and methods are provided for an improvedElectronic Stability Control (ESC) system for a vehicle. An electroniccontrol unit (ECU) is programmed to reduce vehicle lateral skidding byreducing differences between an intended vehicle yaw rate and an actualvehicle yaw rate by applying modifications to operation of the vehiclebrakes and throttle. The ESC system receives inputs from wheel speedsensors, a steering wheel position sensor, a yaw rate sensor and alateral acceleration sensor. The ECS system also receives input thatindicates at least a property of the road upon which the vehicle islocated, wherein the road upon which the vehicle is located isdetermined from a positioning system that uses a map database and theproperty is determined from the map database. The ECS system uses theroad property information in determining when and/or how to modifyoperation of the vehicle to reduce vehicle skidding.

Other systems, methods and features of the invention will be or willbecome apparent to one with skill in the art upon examination of thefollowing figures and detailed description. It is intended that all suchadditional systems, methods, and features be included within thisdescription, be within the scope of the invention, and be protected bythe accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The examples of the invention described below can be better understoodwith reference to the following figures. The components in the figuresare not necessarily to scale, emphasis instead being placed uponillustrating the principles of the invention. In the figures, likereference numerals designate corresponding parts throughout thedifferent views.

FIG. 1 is a diagram showing a vehicle with an Electronic StabilityControl system according to an embodiment.

FIG. 2 is a diagram of components included in a database representationof a road included in the database in FIG. 1

FIG. 3 is a flowchart of process performed by the Electronic StabilityControl system of FIG. 1.

FIG. 4 is a flowchart of an alternative embodiment of the processperformed by the Electronic Stability Control system of FIG. 1.

FIG. 5 is a graph that shows a set of three steering inputs.

FIG. 6 is a graph that shows yaw rate error reduction on banked curve inan ESC system equipped with map data.

FIG. 7 is a graph that shows primary lane deviation peaks in a positivebank at 56 mph.

FIG. 8 is a graph that shows primary lane deviation peaks in a positivebank at 60 mph.

FIG. 9 is a table that that shows banked curve ESC performance metrics.

FIG. 10 is a graph that shows RMS yaw rate error in a positive bank atvarious speeds.

FIG. 11 is a graph that shows RMS slip angle in a positive bank atvarious speeds.

FIG. 12 is a graph that shows primary lane deviation peak in a positivebank at various speeds.

FIG. 13 is a graph that shows minimum event velocity in a positive bankat various speeds.

FIG. 14 is a flowchart of an alternative embodiment of the processperformed by the Electronic Stability Control system of FIG. 1.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS I.Electronic Stability Control System

FIG. 1 shows a diagram of a vehicle 100 that includes a presentembodiment of an Electronic Stability Control system 102. The ESC system102 includes sensors that monitor certain vehicle operations, an ESCprogram that receives inputs from the sensors and determinesmodifications for the vehicle operation and outputs commands toactuators that apply the modifications. More specifically, includedamong the sensors of the ESC system 102 is a steering wheel sensor 108.The steering wheel sensor 108 determines the position (i.e., angle) ofthe vehicle steering wheel 112 and outputs a signal on a continuous orregular basis indicating the steering wheel position.

Also included among the sensors of the ESC system 102 is a yaw ratesensor 116. The yaw rate sensor 116 is located within the vehicle 100.The yaw rate sensor 116 measures the yaw rate of the vehicle 100 andprovides an output signal indicative thereof. The yaw rate sensor 116provides a signal indicating the vehicle yaw rate on a regular and/orcontinuous basis.

The ESC sensors also include wheel sensors 120(1), 120(2), 120(3) and120(4). The wheel sensors 120(1), 120(2), 120(3) and 120(4) measure thespeed (i.e., rotation) of each individual wheel, 122(1), 122(2), 122(3)and 122(4), respectively. Each wheel sensor provides an output signalindicating the respective wheel speed. The wheel sensors 120(1), 120(2),120(3) and 120(4) provide output signals indicating the respective wheelspeeds on a continuous and/or regular basis.

The ESC sensors also include a lateral acceleration sensor 130. Thelateral acceleration sensor 130 is located within the vehicle 100. Thelateral acceleration sensor 130 measures the lateral acceleration of thevehicle 100 and provides an output signal indicative thereof. Thelateral acceleration sensor 130 provides a signal indicating thevehicle's lateral acceleration on a regular and/or continuous basis.

The ESC sensors also include a throttle sensor 136. The throttle sensor136 measures the position and/or operation of the vehicle throttle 138.The throttle sensor 136 provides an output signal indicating thethrottle position and/or operation on a continuous and/or regular basis.

The ESC system 102 may not necessarily include all the types of sensorsindicated above. Alternatively, the ESC system 102 may include differentsensors than those mentioned above, or may include other sensors inaddition to those indicated above.

The ESC system 102 includes an electronic control unit (ECU) 144. Theelectronic control unit 144 may be a microprocessor or other computerhardware device capable of being programmed with software, firmware orotherwise. The electronic control unit 144 meets standard specificationsfor use and operation in vehicles.

The electronic control unit may be an application-specific integratedcircuit (“ASIC”), digital signal processor, field programmable gatearray (“FPGA”), digital circuit, analog circuit, a general processor, orcombinations thereof. In one embodiment, the processor is one or moreprocessors operable to control and/or communicate with the variouselectronics and logic of the associated components or devices.

The electronic control unit 144 runs an electronic stability controlapplication 150. The electronic stability control application 150 is aprogram implemented in software or firmware. The electronic stabilitycontrol application 150 executes program instructions to carry out thefunctions of the Electronic Stability Control system, as explainedherein. The electronic stability control application 150 receives thesignal inputs from the Electronic Stability Control system sensors. Morespecifically, the electronic stability control application 150 receivesthe signal outputs from the steering wheel sensor 108, the yaw ratesensor 116, the wheel sensors 120(1), 120(2), 120(3) and 120(4), thelateral acceleration sensor 130, and the throttle sensor 136.

The ESC system 102 includes actuators that carry out the commandsdetermined by the ESC application 150 to modify operation of certainvehicle systems. As determined by the electronic stability controlapplication 150, the ECU 144 provides signals to one or more hydrauliccontrol units 156 and 158. The hydraulic control unit 156 controlsoperation of an actuator 162 associated with the vehicle throttle 138.The hydraulic control unit 158 controls the operation of actuators166(1), 166(2), 166(3) and 166(4) each of which is associated with abrake associated with one of the respective wheels 122(1), 122(2),122(3), and 122(4). By means of these actuators, the ESC system 102 canactuate wheel brake forces as well as engine throttle to affect thedynamics (i.e., operation and movement) of the vehicle 100.

The ESC system 102 includes an enable/disable switch 170. This switch170 allows the driver to de-activate operation of the ESC system 102.The switch 170 may be mounted on an instrument panel of the vehicle 100.The enable/disable switch 170 is associated with a warning light orother indicator to inform the driver when the ESC system 102 has beende-activated.

In this embodiment, the ESC system 102 also includes a map database 180and a positioning system 184. The map database 180 includes a datarepresentation of the road network including data representing the roadsupon which the vehicle 100 is traveling. In one embodiment, the datarepresentation models each road as a series of road segments, where aroad segment is that portion of a road between intersections (i.e.,where the road connects with another road) or where a road ends.Alternatively, the map database 180 may represent the roads in the roadnetwork in any other manner. The map database 180 may be part of anavigation system, or may be used by a navigation system. However, inother embodiments, the map database is not necessarily used by anavigation system.

The map database 180 is stored in the vehicle on a suitable data storagemedium, such as a hard drive, CD-ROM, DVD, flash drive, or othertangible media suitable to store data. The map database may also belocated remotely from the vehicle and accessed via a wirelesscommunications network. Alternatively, a portion of the map database maybe located in the vehicle and another portion located remotely.

A suitable map database may be provided by a map developer company, suchas NAVTEQ North America, LLC, located in Chicago, Ill.

The map database 180 includes data that represents the road network uponwhich the vehicle travels. In one embodiment, the map database 180includes data that represents the road network throughout an entirecountry, such as the United States. Alternatively, the coverage area maycorrespond to several countries, such as the United States, Canada, andMexico. According to another alternative, the coverage area of the mapdatabase may include only a portion of a country or region within ageographic area, such as a county, state, province, city, metropolitanarea, a regularly-shaped area or an irregularly-shaped area. The mapdatabase 180 may represent all the roads within a covered geographicarea or alternatively, the map database may represent only a portion ofthe roads within an area, e.g., controlled-access roads or high volumeroads.

The positioning system 184 includes hardware and software thatdetermines the position of the vehicle 100 on the road network. Thepositioning system 184 may include a Global Navigation Satellite System(GNSS) unit (such as GPS, Galileo, Glonass or Compass) and/or otherpositioning hardware 186, such as inertial sensors including gyros,accelerometers and inclinometers. The positioning system 184 may alsoinclude the wheel speed sensors 120. The positioning system 184 alsoincludes a positioning application 188. The positioning application 188is a software routine or program that uses information output by thepositioning hardware 186 and the map database 180. The positioningapplication 188 determines the three dimensional position, velocity anddirection of the vehicle along a road segment. The positioningapplication 188 may be installed, run or executed on the same electroniccontrol unit 144 as the Electronic Stability Control application 150, oralternatively, the positioning application 188 may be installed, run orexecuted on a separate processor.

FIG. 2 is a diagram that shows some of the data components that may beincluded in a data representation 190 of a road segment in the mapdatabase 180. Each data representation 190 of a road may include an ID190(1) that uniquely identifies the data record. The data representation190 of a road also includes information 190(2) about the position andshape of the road. The position information 190(2) may include thelatitude, longitude, and altitude of the end points (e.g., nodes) of theroad segment, and possibly, positions along the road segment between theend points (e.g., shape points or continuous equations).

In this embodiment, the representation of the road 190 includesinformation useful for operation of the ESC system 102. The followingare some of the types of information about a road segment that may beincluded in the map database 180. It should be understood that anembodiment of the map database does not need to include all these typesof data. It should also be understood that a map database may includeother types of data, in addition to these. Further, the informationabout a road segment may indicate whether the information applies to theentire represented road segment or a portion of the road segment,whether the described property or feature changes along the representedroad segment, where such change takes place and the property or featureon either side of the change.

Position and Shape (Geometry). The data representation of the road mayinclude information 190(2) that indicates the position and shape(geometry) of the road. The geometry may be indicated for the entireroad segment, or at various points along the road segment. This propertyof the road may be indicated by means of shape points, splines,clothoids, or other means or formulas.

Curvature. The data representation of the road may include information190(3) that indicates the curvature or radius of curvature of the road.The curvature or radius of curvature may be indicated for the entireroad segment, or at various points along the road segment. This propertyof the road may be indicated by means of shape points, splines,clothoids, or other means or formulas.

Height and Slope. The data representation of the road may also includeinformation 190(4) that indicates the height and slope along of theroad.

Speed limit. The data representation of the road may also includeinformation 190(5) that indicates the speed limit along the road. Thisspeed limit information may include whether the speed limit changesalong the road, or whether the speed limit changes at different times orunder different conditions (e.g., school zones)

Lanes and Lane width. The data representation of the road may alsoinclude information 190(6) that indicates the number and location oflanes along the road. The information may also indicate the width ofeach lane.

Road surface. One type of data that may be included is data thatindicates the type of surface of the road. This type of information190(7) may indicate whether the surface is paved, unpaved, cement,blacktop, gravel, grooved, etc.

Shoulders. The data representation of the road may also includeinformation 190(9) that indicates whether shoulders exist along theroad. This information may indicate whether a shoulder exists on bothsides, or only one side of the road. This information may also indicatethe size (i.e., width) of the shoulder, the type of surface of theshoulder, the length along the road that the shoulder is present, aswell as other properties of the shoulder.

Friction. The data representation of the road may also includeinformation 190(10) that indicates the frictional properties of thesurface of the road. This may be indicated by frictional coefficient orother means.

Superelevation. The data representation of the road 190 may also includeinformation 190(11) that indicates the superelevation (cross-slope)along the road.

Black Spots. The data representation of the road 190 may also includeinformation 190(12) that indicates the locations of “black spots” alongof the road. Black spots are locations of higher than normal accidentrates. The black spot information 190(12) may indicate the specificlocation of a black spot along a road segment as well as the kind ofaccident condition associated with the black spot.

Medians. The data representation of the road may also includeinformation 190(13) that indicates whether a median exists along a road.The information 190(13) may indicate the size and composition (e.g.,concrete, landscaped) of the median. The information 190(13) may alsoindicate whether there are any passages through the median and wheresuch passages are located.

Guard rails, medians and other roadside structures. The datarepresentation of the road may also include information 190(14) thatindicates whether any guard rails exist along a road and if so whetherthe guard rails are on the left, right or both sides of the road. Thedata representation of the road may also include any other structuresalong the road.

Other data. The types of data indicated above are not exclusive. Theremay be other kinds of data 190(n) associated with the representation ofthe road.

II. ESC Operation Overview

Electronic stability control (ESC) systems operate by individuallyactuating the wheel brakes, and possibly other vehicle systems such asthe throttle, to induce a yaw moment on the car for the purpose ofimproving stability and performance. For the ESC system to accomplishthis, the system collects information about the car's actual operationand movement, and how the car “should” be operating and moving underideal circumstances. Therefore, a certain amount of instrumentation isused, as well as an on-board dynamics model for making predictions. Acomprehensive description of ESC system operation is given in the BoschAutomotive Handbook (6th Edition, 2004, ed. Robert Bosch GmbH).

As stated above, the ESC system monitors and controls the differencebetween the measured yaw rate of the car and its ideal calculated yawrate. The yaw rate gain equation can be used to estimate the yaw ratewhile traveling on a curve.

$r_{unlimited} = \frac{\delta_{f}v_{x}}{L + {K_{u}v_{x}^{2}}}$

where r is the yaw rate, δ_(f) is the front axle steer angle, ν_(x) isthe longitudinal velocity, L is the wheelbase, and K_(u) is theundersteer gradient.

While this equation considers the particular understeer gradient, K_(u),of the car, it does not take into account the friction of the road.There is a maximum achievable yaw rate that is based on the measuredlateral acceleration and longitudinal speed.

$r_{\max} = {- \frac{\alpha_{y}}{v_{x}}}$

where α_(y) is the lateral acceleration

The formula for maximum yaw rate is implicitly dependent on thecoefficient of friction. Given a friction coefficient of x, the car willbegin slipping sideways when the lateral acceleration reaches x*g. Thus,the acceleration would not exceed the friction coefficient of the road.The revised yaw rate estimate takes the upper bound into account.R _(est)=min(|r _(unlimited) |,|r _(max)|)sgn(r _(unlimited))

An estimate of the yaw rate error is the difference between the measuredvalue from the car's sensor and the estimated yaw rate.r _(diff) =r _(meas) −r _(est)

However, the estimate does not include the dynamics of the lateralhandling, so there are some lags that contribute to phase differencesbetween the measured and estimated yaw rate. Some experimentation withfiltering and differentiating the yaw rate difference signal results inan acceptable estimate for the purpose of triggering ESC activation.

When the yaw rate error exceeds a certain threshold, the ESC systemactivates and brakes are applied to individual wheels to create thedesired yaw moment on the car. Excessive understeer causes the car toleave the intended path towards the outside of the curve; and the actionof the ESC system is to apply the inside rear wheel brake to bring thecar back into the lane. On the other hand, excessive oversteer causesthe car to swerve to the inside of the intended path and possibly spinout. The action of the ESC system in this case is to apply the outsidefront wheel brake to counter the spin. Existing conventional ESC systemsare not actually aware of the driver's intention, or of the shape of theroad. Existing conventional systems react to a developing yaw rateerror. It should be understood that existing conventional systems mayemploy other error signals in addition to yaw rate error, such asestimated slip angle error for example.

III. ESC Operation Flowchart

FIG. 3 is a flowchart that illustrates operation of a present embodimentof the ESC system 102 in FIG. 1. FIG. 3 shows a process 200 performed bythe ESC system 102. In this embodiment, the process 200 includes twocomponent processes. One component process 204 includes the stepsperformed by the ESC application 150 in FIG. 1. This component process204 monitors vehicle operation and outputs commands to effectmodifications to the vehicle operation. The other component process 208includes steps performed by the positioning application 188 in FIG. 1.This component process 208 determines a vehicle position and propertiesof the road upon which the vehicle is traveling. In the presentembodiment, the processes 204 and 208 operate in parallel. Further, in apresent embodiment, the processes 204 and 208 operate continuously andloop, i.e., cycle back to perform their steps over and over while thevehicle is being operated.

Referring first to the positioning process 208, data from thepositioning hardware 184 that indicates the vehicle position is obtained(Step 212). In this step, the input may be in the form of geographiccoordinates. Alternatively, the input may include data from inertialsensors, wheel speed sensors (i.e., 120), or other hardware.

Using the information obtained from the positioning hardware 184, thepositioning process 208 matches the vehicle position to a location alonga road segment (Step 216). The process 208 uses the map database 180 forthis purpose. The map matching step 216 may determine the exact currentlocation of the vehicle along a road segment, including the directionthe vehicle is heading and the lane in which the vehicle is located. Inan alternative embodiment, the map matching step 218 may also determinea portion of the road network ahead of the vehicle. Determining aportion of the road network ahead of the vehicle may be accomplishedusing processes disclosed in U.S. Pat. Nos. 6,405,128 and 6,735,515, theentire disclosures of which are incorporated by reference herein. Usingthe technology disclosed in these patents and information from the mapdatabase, a data model of the road ahead of the vehicle is created,maintained and updated. The data model of the road extends out to anextent or threshold. The extent may be based on distance, time totravel, posted speed limits, actual vehicle speed, or other factors. Forexample a distance extent may be 0.5 km, 1 km, 2 km, or other distances.If an intersection is located within the extent, the data model of theroad ahead of the vehicle may also contain information on the multiplepossible roads onto which that the vehicle may travel. The data modelmay also identify a most-likely-path, which would be the one road ofmultiple possible roads that the vehicle is most likely to travel onto.Determination of the most-likely-path is described in the aforementionedpatents.

Next, the positioning process 208 includes a step of determining a roadproperty associated with the determined vehicle position (Step 220). Thepositioning process 208 obtains data from the map database 180 for thispurpose. The road property may include any of those properties discussedabove in connection with FIG. 2.

Once the positioning process determines the applicable road propertyassociated with the location of the vehicle (and possibly ahead of thevehicle), it outputs this information to the ESC application 150 (Step226). Then, the positioning process 208 starts again with the step ofgetting input indicating the vehicle position from the positioninghardware 184 (Step 212, again). The steps in the position process 208are repeated while the vehicle is operated.

Referring now to the ESC process 204 in FIG. 3, this process 204includes the step of obtaining inputs from the ESC sensors (Step 230).As stated above, the ESC sensors include those that indicate the actualvehicle operation and the intended vehicle operation. More specifically,these sensors may include the steering wheel sensor 108, the yaw ratesensor 116, the wheel speed sensors (collectively, 120), the lateralacceleration sensor 130 and the throttle sensor 136.

There are different ways that the ESC process 204 can use the roadproperty information. Therefore, there are different stages in the ESCprocess 204 at which the road property information may be obtained. Inthe embodiment shown in FIG. 3, the ESC process 204 obtains the roadproperty information that was output in step 226 of the positioningapplication process 208 (Step 234) before the step of determining theactual vehicle yaw rate and the intended vehicle yaw rate (Step 240).The ESC process 204 may obtain the road property information at thisstage when the road property information is used as part of the step ofdetermining the actual or intended yaw rates. Some examples of using theroad property information in this way are described in sections thatfollow.

Next the ESC process 204 compares the actual and intended vehicle yawrates (Step 244). If the difference between the actual vehicle yaw rateand the intended vehicle yaw rate does not exceed a threshold, the ESCprocess 204 returns to the step of obtaining the sensor inputs (Step230, again) and the process 204 continues from there.

Returning to Step 244, if the difference between the actual vehicle yawrate and the intended vehicle yaw rate exceeds a threshold, the ESCprocess 204 determines an appropriate modification to the vehicleoperation (Step 248). Then, the ESC process 204 provides an appropriateoutput (i.e., via the HCU) to the vehicle actuators, i.e., the throttleactuator 162 and/or the individual brake actuators 166(1), 166(2),166(3), and 166(4) to effect the determined operation of the vehicle(Step 252). Then, the process 204 returns to obtain inputs from thesensors (Step 230) and the process continues as before.

As mentioned above, there are different ways that the ESC process 204can use the road property information and accordingly there aredifferent stages in the ESC process 204 at which the road propertyinformation may be obtained. FIG. 4 shows an alternative embodiment ofthe ESC process 204. The ESC process 204 in FIG. 4 includes some of thesame steps as the ESC process shown in FIG. 3, and like steps areindicated by the same numerals.

In FIG. 4, the ESC process 204 obtains the road property informationfrom the positioning application process 208 after the determination ismade (in Step 244) that the difference between the actual vehicle yawrate and the intended vehicle yaw rate exceeds a threshold (Step 260).The ESC process 204 in FIG. 4 may obtain the road property informationat this stage when the road property information is used as part of thestep of determining the appropriate modification to make to the actualvehicle operation. Some examples of using the road property informationin this way are described in sections that follow below.

In another alternative method of operation, the ESC process may use theroad property information both for determining the actual or intendedyaw rates and for determining the appropriate modification to make tothe actual vehicle operation. (This method would be a combination ofFIGS. 3 and 4). In this alternative method of operation, the roadproperty information may be obtained before the step of determining theactual or intended yaw rates and then retained for subsequent use thestep of determining the appropriate modification to make to the actualvehicle operation. Alternatively, the road feature information may beobtained separately for these two functions.

IV. ESC Operation with Digital Map

As stated above, map information about features, properties or otheraspects of the road network, such as those described in connection withFIG. 2, can be used by the ESC system in different ways. Some of theways that map information can be used in the ESC system are describedbelow.

Road geometry. As explained above, ESC systems use various sensorinputs, such as sensors associated with the steering wheel and throttle,to determine the driver's intention. Map information that indicates theroad geometry, i.e., the shape of the road, including the shape of theroad at the specific location of the vehicle as well as the shape of theroad immediately ahead of the vehicle, can be used to help determine thedriver's intention. For example, if map data indicates that the roadupon which the vehicle is traveling is curved to the left, it would beexpected that operation of the steering wheel by the driver to cause thevehicle to curve to the left would be intentional. On the other hand, ifmap data indicates that the road ahead of the vehicle is straight andthe driver operation causes the vehicle to curve to the left, the ESCsystem could assume that the operation of the steering wheel might beunintentional (e.g., the driver is drowsy), or that the driver isattempting to avoid something. In either case, the ESC system mayoperate differently. For example, when the vehicle is being operated tonot follow the path of the road geometry, the ESC system would modifythe vehicle operation more aggressively.

In addition to being used to determine a driver's intention, map datathat indicates the road geometry can also be used by the ESC system aspart of the vehicle recovery strategy. For example, the ESC system canfactor in an upcoming curve radius into a vehicle recovery strategy. Asan example, if the ESC determines that the vehicle should follow acurved path and the map data indicates that the road ahead is sharplycurved, the ESC recovery strategy may further modify the vehicleoperation to include speed reduction, i.e., through use of the throttleactuator.

Slope. Map data that indicates the slope of the road may be used by theESC system. For example, the readings from the sensors can be adjustedto account for any slope of the road, and to rectify a temporarycorruption of any other involved sensor due to an instantaneous changein slope. Further, the slope of the road can be used to determine thedriver's intention. For example, if the vehicle is approaching adownhill slope and the brakes are applied, the ESC system may assumethat application of the brakes was intentional. Likewise, map data thatindicates the slope of the road may be used by the ESC system todetermine a recovery strategy. For example, if the vehicle is headingdownhill, the ESC recovery strategy may apply the brakes with greaterforce and sooner compared to when the vehicle is heading uphill.

Speed limit. Map data that indicates the speed limit of the road at andahead of the vehicle position may be used by the ESC system. Forexample, if the vehicle is being operated above the posted speed limit,the ESC system may assume that speeding accounts for differences betweenactual vehicle operation and intended vehicle operation and modify thevehicle operation accordingly. For example, under these circumstances,the ESC system may favor adjusting the throttle to slow down the vehiclerather than adjusting the brakes to avoid slippage.

Lanes. Map data that indicates the number, location, direction and widthof lanes may be used by the ESC system to determine a driver's intentionand to determine how to modify vehicle operation, if needed. Forexample, a rapid swerve while a vehicle is being driven in a right handlane may indicate a driver's attempt to avoid an obstacle directlyahead. A recovery action commanded by the ESC system might avoidallowing the vehicle to travel into a lane moving in the oppositedirection.

Surface. Map data that indicates the road surface at and ahead of thevehicle position may be used by the ESC system. For example, if thevehicle is being operated on a gravel surface or is approaching a gravelsurface, the ESC system would begin correcting slippage sooner and moreaggressively than on a concrete or asphalt surface.

Friction. Map data that indicates the friction of the road at and aheadof the vehicle position may be used similarly as the surface informationas noted above.

Shoulders. Map data that indicates the presence, size and surfacecomposition of a shoulder of the road may be used by the ESC system. Forexample, if the vehicle is about to slip off the road onto a shouldercomposed of loose gravel, the ESC system would apply more aggressivecorrection than if the shoulder were composed of a more solid material.In another example, if the map data indicates that no shoulder exists atall, the ESC system would increase its recovery action to prevent thevehicle from travelling off the road.

Guard rails and Center Medians. Map data that indicates the presence ofguard rails and center medians along the road may be used like theshoulder information. above.

Superelevation. Map data that indicates the superelevation (banking)along a road may be used by the ESC system. The degree of superelevationdirectly affects whether and how much a vehicle will slip in a curve.Therefore, the more positive superelevation a road has, the lessaggressively the ESC would have to operate to enhance the driver'scontrol of the vehicle. For example, if a vehicle is slipping in a curveand the map data indicates that the superelevation of the road increasesdirectly ahead of the vehicle position, the ESC system may operate lessaggressively because the road's superelevation would be expected tocontribute to reducing slippage.

V. Example

A test analysis was conducted by National Advanced Driving Simulator(NADS) at the University of Iowa. The purpose was to test and evaluatethe benefits of digital map technology applied to the design of ESCsystems. The specific map attributes chosen were road curvature, roadwidth, bank angle, and road slope. The NADS-developed model only modelsESC and not ABS nor TCS.

This test focused on the performance of the ESC system itself byremoving a live driver from the loop and providing a repeatable set ofpre-recorded driver responses in the form of a double-lane-changemaneuver stimulated by a sudden lane incursion event. Thus theexperiments were run in an offline mode that did not require a humandriver. The map data from a NAVTEQ ADAS Research Platform (ADAS RP) wasintegrated into the NADS MiniSim PC-based simulator; and the simulationswere controlled by steering and speed controllers with steering eventsinjected as disturbances.

The driver inputs used in the simulations were extracted from previousNADS ESC research that did use human drivers and elicited avoidancesteering maneuvers. These steering inputs were applied to a variation ofroad types at a variety of speeds. A basic cruise control maintained thevehicle speed, while a lane tracking steering controller provided thebasis of a closed-loop steering system, with the recorded steeringprofiles treated as disturbances. A set of three of the steering inputsis shown in FIG. 5, and illustrates the degree of variation in thesteering command between drivers reacting to the same event. A total of67 right incursion steering events were collected for use in this study.

An experimental design was created to evaluate the benefits of enhancingESC systems with digital map information. The conditions include abaseline (no ESC), standard ESC, and digital map-enhanced ESC (possiblymore than one), as well as the OEM ESC system for validation purposes.Furthermore, within each condition various speeds were run; and allconditions and speeds were simulated in multiple scenarios. A frictioncoefficient of 0.3 was used on all roads to obtain the clearest contrastbetween the conditions.

Four scenarios were designed from situations in which digital mapinformation could be expected to improve the performance of an ESCalgorithm. They include: a positively banked right curve, a negativelybanked right curve, a downhill slope, and a narrow road. The completeset of steering inputs was run in each scenario. Additionally, driveswith no steering disturbance were done at higher speeds on the curvescenarios. Triggers were embedded in each scenario so that the steeringdisturbance occurred at the same spot on the road regardless of thespeed. The independent variables were ESC condition, scenario, andvehicle speed. Three of the dependent variables which revealedinteresting comparisons between conditions were lane deviation, yaw rateerror, and slip angle error.

The experiment used a three-factor factorial design. Each run wasconducted 67 times, with steering inputs taken from actual driverrecordings. The set of steering inputs was fixed, the dynamics model wasdeterministic, and there was no human interaction in the study. Thisremoved the requirements for randomization and blocking that arenormally built into the design of experiments. Due to the large numberof simulation runs that resulted from this design, batch processes andautomated analysis scripts were developed to help provide efficient anderror-free processing.

Path Follower

A path-following steering controller was developed to automaticallyprovide appropriate steering inputs during a scenario drive. The pathwas obtained by driving manually through each scenario at a low speedand saving the vehicle position trace as path data. The study driveswere then controlled to follow the pre-recorded path at various speedsand in the presence of steering disturbance inputs. The overall steeringsignal was a combination of a feed-forward component, a feedbackcontroller component, and a disturbance component. The path followerapproach was similar to the goal point approach described by Sidhu etal. (A. Sidhu, D. R. Mikesell, D. A. Guenther, R. Bixel and G.Heydinger, “Development and Implementation of a Path-Following Algorithmfor an Autonomous Vehicle,” vol. SP-2138, 2007), except for the openloop components which were indexed by distance travelled, with thedisturbance input being triggered also by distance.

Road Selection

Two road sections were located in the North American NAVTEQ databasethat would satisfy the requirements of the scenarios. Road curvature andslope attributes were used from the digital map database, while roadwidth and bank angle were synthesized.

The first road segment chosen was a section of Dubuque Street north ofIowa City, in North Liberty. This section of road was used for both ofthe curve scenarios, as well as the road width scenario. The directionof travel in each of these scenarios is northwest to southeast. Bothcurve scenarios are mapped to the same physical curve in the database,but synthesized with different bank angles. The first curve to theright, travelling in the southeasterly direction, was used for thecurves. Its radius of curvature goes down to a minimum of approximately400 ft. The second road was a section of Highway 61 near Dubuque. Thisroad has a grade of 6% in places, the maximum allowed on most roads,making it a good candidate for the slope scenario.

Data Analysis

A complete set of engineering data was collected from each run. Thisdata included vehicle dynamics variables, control signals, digital mapattribute variables, and ESC system variables. The large amount ofengineering data was reduced to obtain a descriptive set of metrics thatcharacterize the performance of the vehicle with the ESC system in thecondition being tested. A statistical analysis was then applied to thereduced data to test for significant differences in the performance ofthe ESC systems, with the baseline condition of “no ESC” as a controlcase.

The main dependent variables were based on the lane deviation, yaw rateerror, and slip angle. Lane deviation refers to the magnitude of theexcursion from an “ideal” path that was manually recorded at a slowspeed. The lane deviation decay is the slope between the two measuredlane deviation peaks, calculated as the difference between the peaksdivided by the distance, in feet, separating the peaks. Therefore, anegative decay represents stability, while a positive decay indicatesincreasing oscillations and instability. The slip angle is the anglebetween the wheel steer angle at the front axle and the velocity vectorof the car. The yaw rate error is the difference between the yaw ratepredicted by the ESC car model and the yaw rate measured from thevehicle dynamics.

The analysis utilized SAS statistical software where the mixed methodsprocedure for analysis of variance was used (SAS Institute, Inc., “SASOnlineDoc, version 8”, http://v8doc.sas.com/sashtml, 1998). The mixedmethods procedure assumes a normal distribution. All statistical modelsincluded the ESC algorithm as the independent variable with each of thedependent variables described above. The same analyses were completedacross all road types for each speed condition. A significance level ofp=0.1 was used with significant p-values shaded in the results tables.

Results

Many curved roads have a bank angle that tilts the car towards theinside of the curve, referred to as a normally banked or positivelybanked road, to differentiate it from the negative bank that was alsotested in the study. The positive bank scenario uses a curve with aradius of 400 feet and a bank of 3%. A set of 67 steering inputs wasused at speeds from 54 mph to 60 mph. The steering disturbance wastriggered near the point of maximum curvature. The set of ESC conditionsruns consist of the following: baseline (No ESC), OEM ESC (OEM), NADSESC model (ESC), and ADAS enhancement (ADAS). Additional runs were madeat higher speeds with no steering disturbance. In addition to theformerly mentioned ESC conditions, an additional ADAS enhancement wasadded with curve look-ahead (ADAS LA) introducing engine throttlemodulation.

All tested speeds produced statistically significant differences in RMSyaw rate error between the nominal ESC algorithm and the ADASenhancement. The yaw rate error was plotted for the four speeds thatwere statistically analyzed in FIG. 6. The error bars mark the 95%confidence intervals for the data sets that each bar represents.

The ADAS enhancement is better able to control the yaw rate error in thesimulated range of speeds. Referring to the peak value of the first lanedeviation for each of the steering inputs, in each plot, the independentsamples have been reordered monotonically using the ESC condition. Thevertical axis has been limited and may not show some values if theyexceed the axis limit. Generally, larger lane deviation magnitudes areassociated with larger and/or quicker steering inputs. It is evidentthen that the 67 steering inputs represent a wide range of driversteering patterns.

A typical modern road would be expected to have 12-foot-wide lanes and10-foot-wide shoulders. Assuming a six-foot-wide car, one may estimatethat a tire would leave the road on the right at a lane deviation ofthree feet and leave the shoulder on the right at a lane deviation of 13feet.

FIG. 7 shows the peak lane deviations at 56 mph. The ADAS enhancementreduced many of the values by two feet or more. A few samples can beidentified in which the ADAS condition actually prevented lane orshoulder departure while the ESC condition did not. FIG. 8 shows thepeak lane deviations at 60 mph. The values quickly exceed the limit ofthe chart axis, but a clear trend of ADAS effectiveness is observed. Thelarger improvements are observed for the more severe cases.

A statistical analysis was done using the RMS yaw rate error, RMS slipangle, RMS lane deviation, and lane deviation decay. All speeds showstrong statistical differences in yaw rate error between the ESC andADAS conditions, as shown in Table 1 in FIG. 9. Moreover, there appearsto be a trend towards significance as the speed increases. The otherthree measures also show trends towards significance as the speedincreases, though the lane deviation measures weaken some at 60 mph.

Curve Look-Ahead

Four higher speeds were run without any steering disturbance applied. Asa result, no statistical analysis is necessary for these runs. Anadditional ADAS enhancement, ADAS LA was added, in which the enginethrottle was cut by the system when it anticipated an approaching curve,and the predicted lateral acceleration would be excessive.

FIGS. 10 and 11 show the yaw rate error and slip angle for the fourspeeds. A clear ranking is evident from these charts and places the ADASLA enhancement ahead of the rest. FIG. 12 shows the peak lane deviationfor the four speeds. Again, the ADAS LA enhancements show significantimprovement in lane deviation performance. The primary reason for theimprovement in ADAS LA performance can be traced to the throttleoverride's ability to mitigate the speed of the car coming into thecurve, as shown in FIG. 13.

CONCLUSIONS

Reasonable and conservative adaptations were implemented to atraditional ESC system to take into account additional road informationavailable from a digital map system. These modifications were testedwith 67 real-world steering inputs, at various speeds, and on differenttypes of roads on which improved performance may reasonably be expected.In all cases, the RMS yaw rate error was significantly reduced using theADAS-enhanced algorithm over the traditional ESC system with generallyimproved lane deviation as well. In the opinion of the researchers, thechallenge in achieving significance in some of the measures is caused,in large part, by the wide range of severity in the steering inputs,resulting in large variances in the data. Larger sample sizes would beexpected to increase the significance of the results.

ADAS enhancements alter the trade-off between ESC performance and driver“feel” by moving the tuning of a typical algorithm to a more aggressivestance. Therefore, it is not surprising that the benefits ofADAS-enhanced ESC are apparent in more extreme maneuvers and at higherspeeds that may challenge both drivers and traditional ESC systems.Digital map data can help the ESC system identify dangerous drivingenvironments and make the system more aggressive in a timely or evenpredictive fashion.

The thrust of this research was to make use of traditional ESC controltechniques and adjust the tuning in response to digital map attributedata; however, the use of ADAS technology opens many new opportunitiesto the algorithm designer. Knowledge of the road curvature allows thesystem to make estimations about driver intent by comparing the knowncurvature with the predicted vehicle path calculated from the car'ssteering input and/or yaw rate. If there is data that indicates thedriver is not following the designated roadway path, the system mayattempt to classify the driver's responses as emergency avoidance ordriver impairment, and respond with an appropriate control strategy (seefor example (I. Dagli, M. Brost and G. Breuel, “Action Recognition andPrediction for Driver Assistance Systems Using Dynamic Belief Networks,”Agent Technologies, Infrastructures, Tools, and Applications forE-Services, pp. 179-194, 2009)).

Digital map data can beneficially be used to adjust the authority of theESC system in response to road conditions and vehicle speed. Parametertuning that provides effective yet comfortable ESC performance in normalsituations is different from a tuning that would be more helpful indangerous situations. Map-enhanced ESC has been shown to provide asignificant performance boost in certain realistic situations.

FIG. 14 is another flowchart that shows an alternative embodiment of theprocess performed by the Electronic Stability Control system of FIG. 1.The ESC process 204 in FIG. 14 includes some of the same steps as theESC process shown in FIG. 3, and like steps are indicated by the samenumerals.

FIG. 14 shows a modification to the ESC process 204 in which the roadproperty obtained from the positioning process 208 is used to determinethe value of the threshold, T (Step 288). This threshold T is then usedin the step in which the actual yaw rate is compared to the intended yawrate (Step 244). As an example of how this would be applied, if thevehicle were being operated on a curved road, the threshold T would bereduced accordingly. Then, the ESC application would initiate a recoverymodification at a lower difference threshold between the actual yaw rateand the intended yaw rate compared to when the vehicle was beingoperated on a road that was not curved.

In the methods of operation disclosed above, fewer or more steps or actsmay be provided, and a combination of steps may be provided. Also, thesteps or acts may be performed in the order as shown or in a differentorder. The method is implemented by the system and/or devices describedherein or by different devices or systems. One or more steps orprocesses of the method may be fully or partially automated (such as viaa computer or algorithm).

The logic, data, software, or instructions for implementing the systems,processes, methods and/or techniques discussed above are provided oncomputer-readable storage media or memories or other tangible media,such as a cache, buffer, RAM, removable media, hard drive, othercomputer readable storage media, or any other tangible media. Thetangible media include various types of volatile and nonvolatile storagemedia. The functions, acts, steps, or tasks illustrated in the figuresdescribed herein are executed in response to one or more sets of logicor computer-executable instructions stored in or on computer readablestorage media. The functions, acts or tasks are independent of theparticular type of instructions set, storage media, processor orprocessing strategy and may be performed by software, hardware,integrated circuits, firmware, micro code and the like, operating aloneor in combination. Likewise, processing strategies may includemultiprocessing, multitasking, parallel processing and the like. In oneembodiment, the instructions are stored on a removable media device forreading by local or remote systems. In other embodiments, the logic orinstructions are stored in a remote location for transfer through acomputer network or over telephone lines. In yet other embodiments, thelogic or instructions are stored within a given computer, centralprocessing unit (“CPU”), graphics processing unit (“GPU”) or system.

The foregoing description of an implementation has been presented forpurposes of illustration and description. It is not exhaustive and doesnot limit the claimed inventions to the precise form disclosed.Modifications and variations are possible in light of the abovedescription or may be acquired from practicing the invention. Note alsothat the implementation may vary between systems. The claims and theirequivalents define the scope of the invention.

We claim:
 1. An improved Electronic Stability Control system for avehicle comprising: a map database that includes data that indicatesproperties of roads in a road network including shapes of roads uponwhich the vehicle travels; a positioning system operably coupled to themap database to determine a position of the vehicle on a road in theroad network; an electronic control unit comprising a processor operableto execute programming functions, wherein said electronic control unitis operably coupled to sensors that indicate a vehicle yaw rate and asteering wheel position, and further wherein said electronic controlunit is operably coupled to the positioning system to receiveinformation indicating the position of the vehicle on the road, andwherein said electronic control unit includes outputs to brake actuatorsassociated with brakes associated with individual wheels of saidvehicle; and an ESC program run on the electronic control unit that usesinputs from the sensors and information from the map database thatindicates a shape of a portion of the road upon which the vehicle islocated that is ahead of the position of the vehicle so as to determinea derived vehicle yaw rate which would result from intended operation ofthe vehicle by a driver of the vehicle and to determine sensitivity todifferences between the derived vehicle yaw rate and an actual vehicleyaw rate to determine whether to provide outputs to the brake actuatorsto reduce differences between the derived vehicle yaw rate and theactual vehicle yaw rate based upon the extent to which the shape of theportion of the road ahead will itself cause a reduction of thedifference between the derived vehicle yaw rate and the actual vehicleyaw rate.
 2. The system of claim 1 wherein the ESC program also usesinformation from the map database that indicates road slope to determinewhether to provide outputs to the brake actuators to reduce differencesbetween the derived vehicle yaw rate and the actual vehicle yaw rate. 3.The system of claim 1 wherein the ESC program also uses information fromthe map database that indicates road width to determine whether toprovide outputs to the brake actuators to reduce differences between thederived vehicle yaw rate and the actual vehicle yaw rate.
 4. The systemof claim 1 wherein the ESC program also uses information from the mapdatabase that indicates road bank angle to determine whether to provideoutputs to the brake actuators to reduce differences between the derivedvehicle yaw rate and the actual vehicle yaw rate.
 5. The system of claim1 wherein the ESC program uses the information from the map databasethat indicates the shape of the portion of the road ahead of where thevehicle is located out to an extent to determine whether to provide anoutput to a throttle actuator associated with a vehicle throttle toreduce a speed of the vehicle.
 6. The system of claim 1 wherein the ESCprogram also uses the information from the map database that indicatesthe shape of the road upon which the vehicle is located to determine arecovery action, wherein the recovery action is implemented by providingoutputs to the brake actuators to reduce differences between the derivedvehicle yaw rate and the actual vehicle yaw rate.
 7. The system of claim1 wherein the ESC program also uses the information from the mapdatabase that indicates the shape of a portion of the road ahead ofwhere the vehicle is located out to an extent to determine a recoveryaction, wherein the recovery action is implemented by providing outputsto the brake actuators to reduce differences between the derived vehicleyaw rate and the actual vehicle yaw rate.
 8. The system of claim 7wherein the recovery action also includes providing outputs to athrottle actuator associated with a vehicle throttle to modify vehiclespeed.
 9. A method of operation for an electronic stability controlsystem in a vehicle, the method comprising the steps of: determining, bya processor, a location of the vehicle along a road in a road network;accessing, by the processor, a map database to determine a shape of theroad ahead of the location of the vehicle along the road; obtaining, bythe processor, outputs from sensors in the vehicle that indicate anactual vehicle yaw rate and a derived vehicle yaw rate which wouldresult from operation of the vehicle in a manner intended by the driver;upon determining that the actual vehicle yaw rate exceeds the derivedvehicle yaw rate by a threshold value, wherein the threshold value isdetermined at least in part by the shape of the road ahead of thelocation of the vehicle along the road, providing, by the processor,outputs to brake actuators to reduce a difference between the actualvehicle yaw rate and the derived vehicle yaw rate based upon the extentto which the shape of the portion of the road ahead itself will cause areduction of the difference between the derived vehicle yaw rate and theactual vehicle yaw rate.
 10. The method of claim 9 further comprisingthe steps of: determining, by the processor, a portion of the roadnetwork onto which the vehicle could travel from the location; andaccessing, by the processor, the map database to determine shapes ofroads in the portion of the road network onto which the vehicle couldtravel from the location; and further wherein the threshold value isdetermined at least in part by the shapes of roads in the portion of theroad network onto which the vehicle could travel from the location. 11.The method of claim 9 further comprising: upon determining that theactual vehicle yaw rate exceeds the derived vehicle yaw rate by thethreshold value, providing, by the processor, an output to a throttleactuator associated with a vehicle throttle to reduce vehicle speed. 12.The method of claim 9 further comprising: accessing, by the processor,the map database to determine road slope at the location of the vehiclealong the road; and further wherein the threshold value is determined atleast in part by the road slope at the location of the vehicle along theroad.
 13. The method of claim 9 further comprising: accessing, by theprocessor, the map database to determine road width at the location ofthe vehicle along the road; and further wherein the threshold value isdetermined at least in part by the road width at the location of thevehicle along the road.
 14. The method of claim 9 further comprising:accessing, by the processor, the map database to determine road bankangle at the location of the vehicle along the road; and further whereinthe threshold value is determined at least in part by the road bankangle at the location of the vehicle along the road.
 15. The method ofclaim 9 further comprising the steps of: determining, by the processor,a portion of the road network onto which the vehicle could travel fromthe location; accessing, by the processor, the map database to determineshapes of roads in the portion of the road network onto which thevehicle could travel from the location, wherein the threshold value isdetermined at least in part by the shapes of roads in the portion of theroad network onto which the vehicle could travel from the location; andproviding, by the processor, an output to a throttle actuator associatedwith a vehicle throttle to reduce vehicle speed upon determining thatthe actual vehicle yaw rate exceeds the derived vehicle yaw rate by thethreshold value.
 16. The method of claim 9 further comprising: using, bythe processor, the information from the map database that indicates theshape of the road upon which the vehicle is located to determine arecovery action; and implementing, by the processor, the recovery actionby providing outputs to the brake actuators to reduce differencesbetween the derived vehicle yaw rate and the actual vehicle yaw rate.17. The method of claim 9 further comprising: using, by the processor,the information from the map database that indicates the shape of aportion of the road ahead of where the vehicle is located out to anextent to determine a recovery action; and implementing, by theprocessor, the recovery action by providing outputs to the brakeactuators to reduce differences between the derived vehicle yaw rate andthe actual vehicle yaw rate.
 18. The method of claim 17 furthercomprising: providing, by the processor, output to a throttle actuatorassociated with a vehicle throttle to modify vehicle speed.